It means that the source code (code written in the computer programming language) for the specified application is available for reference or for customization.
Open source software (OSS) is defined as computer software for which the source code and certain other rights normally reserved for copyright holders are provided under a software license that meets the Open Source Definition or that is in the public domain. This permits users to use, change, and improve the software, and to redistribute it in modified or unmodified forms. It is very often developed in a public, collaborative manner. Open source software is the most prominent example of open source development and often compared to user-generated content. The term open source software originated as part of a marketing campaign for free software. A report by Standish Group states that adoption of open source software models has resulted in savings of about $60 billion per year to consumers.
Open Source software is a software product that the creator has published the source code of the software so anyone can view and edit it. One example is the Chromium web browser Google Chrome is built on.